A vertical cavity surface-emitting laser (VCSEL) is a semiconductor laser emitting a light beam perpendicular from the top of its substrate surface. Compared to a typical edge-emitting semiconductor laser, the VCSEL may have several advantageous characteristics such as a low price, low power consumption, compact and high performance, and easy to integrate two dimensionally.
A typical surface-emitting laser includes a resonator structure having a resonator region with an active layer, and an upper reflector and a lower reflector respectively at an upper part and a lower part of the resonator region (e.g., Patent Document 1). In the aforementioned surface-emitting laser, the resonator region is formed in a predetermined optical thickness in order to acquire an oscillation wavelength λ. With this configuration, a light beam oscillates at the oscillation wavelength λ in the resonator region. In the aforementioned surface-emitting laser, the upper and lower reflectors are formed of materials with mutually different refractive indices which are alternately layered. That is, a material with a low refractive index (low refractive material) and a material with a high refractive index (high refractive material) are alternately layered such that the low refractive material and the high refractive material have an optical thickness of λ/4. With this configuration, the low refractive material and the high refractive material may be able to acquire high reflectance at the wavelength λ. Alternatively, there is proposed a surface-emitting laser formed of elements having different wavelengths within a chip (e.g., Patent Documents 2 to 4, and 6).
In the meantime, there is disclosed an atomic clock (atomic oscillator) that is capable of providing an extremely accurate time, and a technology for reducing the size of the atomic clock has been extensively studied. The atomic clock is an oscillator that oscillates based on transition energy of electrons in alkali metal atoms. Specifically, the transition energy of electrons in alkali metal atoms without disturbance is capable of acquiring extremely accurate values, and hence atomic oscillators may acquire several digits higher frequency stability compared to quartz oscillators.
There are several types of atomic clocks; however, a coherent population trapping (CPT) based atomic clock, among other types, may have three digits higher frequency stability compared to the quartz oscillator, and may, in future, be formed in ultra-compact size and consume ultra-low electric power (e.g., Non-Patent Documents 1 and 2, Patent Document 5).
FIG. 1 illustrates a structure of a CPT-based atomic clock. As illustrated in FIG. 1, the CPT-based atomic clock includes a laser element 910, a cell 940 configured to encapsulate alkali metal, and a receiving element 950 configured to receive a laser beam having passed through the cell 940. The CPT-based atomic clock having such a configuration modulates the laser beam and simultaneously transitions and excites two electrons in alkali metal atoms with sidebands appearing two sides of a carrier wave having a specific wavelength. The transition energy remains unchanged. When wavelengths of the sidebands match the wavelength of the transition energy, a clearing response that reduces optical absorption in the alikali metal may occur. Thus, in the CPT-based atomic oscillator, the wavelength of the carrier wave is adjusted such that the optical absorption is reduced in the alikali metal while a signal detected by the receiving element 950 is fed back to the modulator 960 and hence the modulator 960 adjusts the modulation frequency of the laser beam emitted from the laser 910. Note that in the CPT-based atomic clock, the laser beam emitted from the laser element 910 is applied to the cell 940 encapsulating the alkali metal via a λ/4 wavelength plate 930.
As a light source for such an ultra-compact atomic clock, a surface-emitting laser having an ultra-compact size, exhibiting ultra-low electric consumption power and high wavelength quality may be preferable. Further, as the light source for the ultra-compact atomic clock, the carrier wave exhibiting high wavelength accuracy of ±1 nm with respect to the specific wavelength (e.g., Non-Patent Document 3).